1. Field of the Invention
The present invention relates to a detection device for detecting an element impedance of a sensor element capable of detecting a concentration of oxygen contained in target detection gas such as exhaust gas emitted from an internal combustion engine, and also relates to a sensor unit equipped with the detection device.
2. Description of the Related Art
There have been proposed various conventional techniques for detecting an element impedance of a sensor element assembled in a gas sensor such as an air/fuel sensor (A/F sensor) on the basis of detection results of a current change value and a voltage change value when a supplied voltage or current is temporarily and/or continuously changed. Such an A/F sensor has a sensor element made of solid electrolyte.
The presence of noise which is overlapped or superimposed on an impedance detection signal output from the sensor element in the gas sensor decreases the detection accuracy of the sensor element. Various conventional techniques have been proposed so as to eliminate noise components from an impedance detection signal. There are various types of noise which affect such an impedance detection signal, for example, heater noise generated when one or more heater parts are turned on and off in order to activate the sensor element, and ignition noise generated when an internal combustion engine is ignited.
A description will be given of the occurrence of incorrect detection caused when an element impedance of a sensor element in a gas sensor is detected with reference to FIG. 8A to FIG. 8F.
FIG. 8A to FIG. 8F are timing charts showing waveforms of various signals when an element impedance of a sensor element is detected during a predetermined period of time.
In particular, FIG. 8A is a timing chart showing a waveform of an alternating-current (AC) voltage which is temporarily supplied to the sensor element during a predetermined period of time when an element impedance of the sensor element is detected. FIG. 8B is a timing chart showing a change of the element current which flows in the sensor element when the sensor element is detected shown in FIG. 8A.
FIG. 8C is a timing chart showing a peak hold value of a peak hold circuit.
A detection device calculates the element impedance of the sensor element in the gas sensor on the basis of the peak hold value which is held in the peak hold circuit.
Because a control device (not shown) executes a duty control for repeatedly turning on and off the heater parts formed in the gas sensor, there is a possibility of the timing of detecting the element impedance agreeing with the timing of turning on and off the heater parts.
FIG. 8D is a timing chart showing the turning on and off of heater parts by supplying electric power to the heater parts. FIG. 8E is a timing chart showing an element current with heater noise generated by turning on and off of the heater parts. FIG. 8F is a timing chart showing an error of a peak hold (P/H) value with noise.
As shown in FIG. 5D, when the heater parts in the gas sensor are turned on, namely, electric power is supplied to the heater parts, heater noise is generated and superimposed on an element current. This element current flows in the sensor element in the gas sensor, and supplied to the control device. In this case, as shown in FIG. 8F, the peak hold circuit holds the peak hold value with an error which is different from its true P/H value. As a result, the control device calculates an element impedance of the sensor element with error.
For example, Japanese patent laid open publication No. JP 2006-343317 discloses a configuration of a conventional technique in which a filter means (a low pass filter) is placed at a front stage of a hold circuit in order to eliminate noise from a detection signal regarding an element impedance of a sensor element. The hold circuit holds a voltage changed value when an element impedance of a sensor element is detected. This conventional technique can eliminate noise which is superimposed on the detection signal regarding an element impedance of a sensor element at a stage before the voltage hold circuit holds a voltage changed value in addition to noise which is superimposed on the detection signal of the element impedance. This makes it possible to prevent the accuracy of detecting the element impedance of the sensor element from being decreased, where the decrease of the detection accuracy is caused by incorrect detection of the voltage hold value caused when the voltage hold circuit holds the voltage change value.
However, in the filter means placed at a front stage of the voltage hold circuit in the configuration disclosed in JP 2006-343317, the filter means eliminates a changed response value of an element current in addition to noise. This shifts the voltage change value held in the voltage hold circuit from its true value. Accordingly, it is difficult for the conventional configuration disclosed in JP 2006-343317 to correctly and adequately eliminate noise while maintaining the voltage changed value.
Still further, in order to avoid incorrect detection of the element impedance of a sensor element of a gas sensor caused by heater noise, it is effective for the timing of detecting the element impedance of the sensor element to be shifted from the timing of turning on and off the heater parts. However, this technique is required to have: (a) a sensor control circuit (sensor control IC circuit); and (b) the function of calculating an impedance value of the sensor element on the basis of an output signal of the sensor control circuit, and the function of executing the timing control between the sensor control circuit and a microcomputer capable of controlling the timing to supply electric power to the heater parts. The control device is required to have a complicated configuration and operation in order to execute these functions.